Microbial metabolites have been obtained from bacterial fermentations which inhibit vitamin B12 function in one or more of the following ways: 1) They reverse the vitamin B12 stimulated growth of bacteria and protozoa; 2) They are cytotoxic and this cytotoxicity (to L-1210, P-388, and/or KB cells in tissue culture) can be reversed by inclusion of larger amounts of B12 in the culture media; 3) They inhibit the binding of vitamin B12 to transcobalamin II (as found in Cohn's fraction III of human serum), the B12 binding proteins in ascitic fluid taken from mice bearing Ehrlich ascites tumors, and the B12 binders in human gastric juice and porcine intrinsic factor; and 4) They influence bacteria to produce more vitamin B12 to counteract the inhibitory effects. 27 cultures have been selected (from 2,500 examined) which produce metabolites with one or more of the above activities. We propose to isolate in pure form the 'active materials' from these bacteria and evaluate their biological activities to demonstrate the above properties. We will also make arrangements to have the purified materials evaluated as inhibitors of L-1210 tumors in mice and to determine whether any of these materials can serve as agents to induce vitamin B12 deficiency in microorganisms and animals without killing the test system.